Equations were determined for the calculation of the second stoichiometric (molality scale) dissociation constant (K-m2) of phosphoric acid in buffer solutions containing sodium dihydrogen phosphate, sodium hydrogen phosphate, and sodium chloride and the corresponding systems where mixed potassium and sodium salts were used from the determined thermodynamic values of this dissociation constant (K-a2) and the molalities of the components in the solutions. These equations were based on the single-ion activity coefficient equations of the Huckel type. The parameters of the phosphate ions for these equations and the thermodynamic values of the second dissociation constant of this acid at various temperatures were determined from the Harned cell data of Bates and Acree (1943 and 1945), and these data extend at temperatures from (0 to 60) degrees C up to ionic strengths of about 0.5 mol kg(-1). All calculations from these data were completely revised, and all parameters estimated seem to depend in a simple way on the temperature. The interaction parameters between hydrogen and chloride ions were taken from results of a previous HCl paper (2002). The parameters resulted from interactions between hydrogen ions and sodium or potassium ions and from interactions between sodium or potassium ions and chloride ions were taken from our recent studies (2003 or 2005) where these parameter values were determined from the Harned cell data measured by Harned (1935) or Harned and Hamer (1933) in HCl solutions with NaCl or KCl, respectively. In the present paper, the resulting simple equations were successfully tested with the data used in the parameter estimation. In the second part of this study, these equations were additionally tested with all reliable data found in the literature from this dissociation reaction in NaCl and KCl solutions.